The technical field of this invention is that of nondestructive materials characterization, particularly as it applies to postweld and in-process weld scanning for quality control, in-process monitoring, and seam tracking using spatially periodic field eddy current sensors.
There is an increasing need for a nondestructive method for assessing the quality of welds between materials, including the detection and characterization of defects. In particular, friction stir welding is becoming more commonly used as a joining technique for a variety of metals, including aluminum, titanium and nickel base alloys as well as steels. The quality of the weld depends upon a variety of factors, including the materials, the rotation rate, feed, positioning, applied pressure from the pin tool, and the penetration ligament. Defects such as cracks, lack of penetration (LOP), and lack of fusion can compromise the integrity of the joint and can lead to component failure.
Weld examinations are currently performed to characterize quality of the welds, qualify a welding procedure or qualify welders. These examinations are performed to detect cracks, lack of fusion, lack of penetration, areas of excessive porosity, or unacceptably large inclusions. Liquid penetrant inspection (LPI) is widely used for detection of surface-connected defects in welded components fabricated from nonmagnetizable materials. In some cases, LPI fails to detect these surface-connected defects, such as in the case of tight cracks, cracks densely filled with foreign matter, or weakly-bonded LOP defects in friction stir welds (FSWs).
For components fabricated from magnetizable materials, such as carbon and low-alloy steels, magnetic particle inspection (MPI) is typically used for detection of surface-connected cracks. Some MPI techniques are claimed to detect cracks that are masked by smeared metal so that the cracks are not directly exposed to the surface. Furthermore, MPI is permitted for inspection through thin coatings typically less than 0.003 in. (0.075 mm) thick. However, MPI is limited in crack detection capability and, for coated surfaces, may require coating removal. Methods are needed to inspect carbon and low-alloy steel components for cracks that are below the MPI detection threshold and for inspections that do not require coating removal. There is also a need to characterize residual stresses in these welds. Other conventional nondestructive testing methods such as conventional eddy current sensing are limited in their sensitivity to small flaws in welds and in their capability to extract spatial information about changes in the weld microstructure and flaw characteristics. The use of conventional eddy current sensing often involves extensive scanning along and across the weld.
Etching with a variety of metallographic etchants is also used to reveal macrostructural or microstructural characteristics of welded joints, including weld metal, heat-affected zone, and base metal. In the case of FSW, which is joining by plastic deformation and stirring below solidus, etching can reveal the dynamically recrystallized zone (DXZ), thermomechanically affected zone (TMZ), heat-affected zone (HAZ) and base metal. Etching of FSWs can also be used as a method for characterizing LOP defects, by revealing the relevant width of the DXZ. For example, as shown in FIG. 1, the DXZ, TMZ and HAZ show up after etching as distinctly different zones permitting direct measurement of the width of the DXZ that has penetrated to the backside of the welded panels. Etching of panels joined by FSW would, in the case of butt welds, reveal these zones on both the front and back sides. Unfortunately, the etching process is time consuming, not practical for inspection of long welds required for large structures, such as spacecraft and aircraft, not environmentally friendly, and often not permitted in production. Methods are needed to inspect these surfaces rapidly and nondestructively.
It is often critical to characterize microstructural variations of metal products such as ingots, castings, forgings, rolled products, drawn products, extruded products, etc. Etching of selected samples is used for this purpose but is not practical or permissible for large surfaces or statistically significant quantities, areas, or lengths. It is definitely not acceptable for 100 percent inspection of these products when information on microstructural variations, including imaging of these variations and their quantitative characterization, is required over the entire surface of a product. Furthermore, etching of large surfaces in components that are suspected to contain local zones that are different due to fabrication problems, service-induced or accident-induced effects is not practical, unless the locations of such zones are known a priori.